Various techniques for digital temperature sensing are heretofore known. Some of these techniques rely on thermal resistors to measure a temperature. The measured temperature is converted into a voltage or current signal, and an analog-to-digital converter (ADC) is utilized for subsequent digital output coding of the voltage or current signal.
FIG. 1 of the attached drawings shows a conventional temperature sensing system, broadly designated at 10. The conventional temperature sensing system 10 consists of a temperature sensor 11, a reference circuit 12, and an ADC 13. In order to be fully compatible with standard digital CMOS fabrication processes, the temperature sensor 11 generally adopts parasitic substrate or lateral bipolar transistors for temperature sensing. The temperature sensor 11 generates a voltage or current which varies with temperature. The reference circuit 12 generates a band-gap reference voltage or current that is generally constant with respect to temperature variation. A voltage or current difference between the temperature sensor 11 and the reference circuit 12 is converted into a digital signal by the ADC 13.
For the temperature sensor 11, bipolar transistors are generally adopted for generating the voltage or current corresponding to the measured temperature. However, the behavior or characteristic of bipolar transistors is hard to handle in the fabrication processes, and a significant difference often exists between the theoretic values and the actual values, making it difficult for mass production. In addition, the characteristic curve of the voltage (or the current) vs. temperature usually has problems caused by its curvature. Therefore, the conventional temperature sensing system 10 needs additional calibration circuits for reducing measurement error. It leads to the increase of both chip area and power consumption. Moreover, the reference circuit 12 still needs to use bipolar transistors, and thus has the same problems faced by the temperature sensor 11. As for the ADC 13, it is the important core of the temperature sensing system 10 for output signal digitization. The resolution of the temperature sensing system 10 is decided by the effective output bits of the ADC 13. For a temperature sensing circuit with 0.1° C. temperature resolution for a temperature range over 100° C., an ADC with more than 10 output bits is required to obtain the necessary resolution, often at the expense of large chip area and high power consumption. Furthermore, the above-mentioned three components all need to be implemented with operational amplifiers (OP-AMP). However, the input offset voltage of OP-AMP seriously limits the performance of the temperature sensing system 10. Consequently, it needs a dynamic offset cancellation circuit to improve accuracy and makes the temperature sensing system 10 more complicated. It further increases the chip cost and power consumption. However, the cost, power consumption, and accuracy are most important specifications for temperature sensing chips.
Therefore, there is a need to develop a new temperature sensing system and method for improving the above-mentioned disadvantage of the conventional system.